Since they were first reported in 1989 Antiferroelectric Liquid Crystals (AFLCs) have been considered very attractive for a number of electro-optic applications, foremost for high-resolution large area displays. In antiferroelectric liquid crystal displays (AFLCDs) the smectic Ca* phase liquid crystal is arranged in a bookshelf geometry, as is illustrated in FIG. 1A. When being in the surface-stabilized state and aligned in so-called bookshelf geometry conventional AFLCs are optically biaxial with their slow principal axis along the smectic layer normal. This gives a dark state when the cell is between crossed polarizers oriented parallel and perpendicular to the layer normal.
When a sufficiently strong electric field ±E is applied across the cell, the anticlinic ground state, with an alternating polarization direction in adjacent smectic layers, is forced to one of the two symmetrically situated synclinic states with the slow axis inclined at ±θ away from the layer normal, as is illustrated in FIG. 1B. This is the so-called field-induced transition from the antiferroelectric (AF) to the ferroelectric (±F) state. The switching occurs in domains and grayscale can be produced by controlling the ratio of bright ferroelectric F domains to dark antiferroelectric AF domains. When the field is taken away the AFLC relaxes back to the AF state. The AFLC is thus monostable, in contrast to the surface-stabilized ferroelectric liquid crystal (SSFLC) which is bistable.
So far, the idea to use AFLC materials in a display mode has been, in principle, to use three states of the optic axis lying in a plane, where the two field-on states have been stabilized by a holding voltage applied all the time. This is the so-called tri-state switching. Small video and large desktop computer prototype AFLC displays have been presented already during the 1990's but they were never commercialized. The main reason for this was the insufficient extinction obtained in the dark state.
AFLC materials are notoriously difficult to align in a high-quality bookshelf structure. Instead AFLC materials typically form an inhomogeneous smectic layer structure with local variations in the slow axis (the effective optic axis) orientation in the cell. These variations in the optic axis orientation cause light leakage in the dark state. One part of the alignment problem is the lack of a nematic phase in AFLC materials. Another is the tendency for the structure to break up under electronic addressing conditions. In the latter situation, the vertical chevron formed at the virgin cooling from the smectic A* to the tilted SmCa* (sometimes via SmC*) is straightened up by the electric field and the AFLC now instead forms a “horizontal chevron structure”.
In an attempt to solve the “dark state problem” Orthoconic Antiferroelectric Liquid Crystals (OAFLC) have been developed [1]-[4]. An OAFLC device features AFLC material satisfying the orthoconic condition. In order to satisfy the orthoconic condition, two properties are typically met. The first is a material property where the tilt angle in the anticlinic AF structure is approximately 45°, such that the directors in adjacent smectic layers are perpendicular. The second is a device property where the AF structure is surface-stabilized such that no trace of the helix is present. This second condition is harder to realize in the AFLC than in the FLC case.
The result is that if these two conditions are satisfied the AFLC changes from positive biaxial to negative uniaxial and with the optic axis perpendicular to the layer normal, and perpendicular to the tilt plane, instead of being along the normal. Thus, the AFLC may be in a uniaxial negative state with an oblate indicatrix. This is called the orthoconic condition or the horizontally surface-stabilized orthoconic AFLC state. The orthoconic darkstate is in principle just as good as the extinction of the polarizers, independent of alignment or misalignment, which is unique. Between crossed circular polarizers even the bright state is insensitive to alignment, which is equally unique.